1. Field of the Invention
The present invention relates to a composite substrate and an elastic wave device.
2. Description of the Related Art
In recent years, in order to improve frequency-temperature characteristics of an elastic wave device, a composite substrate formed by bonding a piezoelectric substrate onto a support substrate having a low coefficient of thermal expansion has been used. A general process for forming elastic wave devices using the composite substrate as described above is performed as follows. First, elastic wave-device electrodes are formed on a surface of the piezoelectric substrate of the composite substrate. The surface of the piezoelectric substrate is partitioned so that many elastic wave devices are to be formed, and the elastic wave-device electrodes are formed using a photolithographic technique on positions of the respective elastic wave devices. Next, the composite substrate is diced along the partitions, so that many elastic wave devices are obtained.
However, when heat is applied to the composite substrate in the process for forming elastic wave devices, a stress is generated by the difference in thermal expansion between the piezoelectric substrate and the support substrate. Hence, the composite substrate is considerably deformed, and the accuracy of the electrode width is degraded, so that the frequency characteristics may be degraded in some cases. In addition, depending on the cases, because of the stress generated at the interface between the piezoelectric substrate and the support substrate, the piezoelectric substrate may be peeled away from the support substrate or may be fractured in some cases. As a method to overcome the problems as described above, a composite substrate having a three-layer structure has been proposed in which a compensating layer having a coefficient of thermal expansion equivalent to that of a piezoelectric substrate is provided on a surface of a support substrate opposite to that in contact with the piezoelectric substrate (Patent Literature 1).